1. Field of the Invention
The present invention relates to a light source for optical communication, and more particularly to a wavelength-tunable laser apparatus.
2. Description of the Related Art
A wavelength-division-multiplexed passive optical network (WDM-PON) employs unique wavelengths assigned to respective subscribers to provide high-speed wideband communication services. The development of an economical WDM light source is essential for implementing WDM-PONs. The wavelength-locked Fabry-Perot laser has been proposed as a source, since it is low-priced, can achieve frequency stabilization by outputting only light of wavelength that coincides with inputted light of a desired power level or more, and improves transmission performance by increasing the SMSR (Side Mode Suppression Ratio). The SMSR represents the ratio of the intensity of the light beam outputted after being amplified to the intensity of the light beams outputted after being suppressed, as will be discussed in more detail below. An external diffraction grating, a fiber Bragg grating, and an FP filter are used to induce a wavelength-locked phenomenon in the FP laser.
FIG. 1 depicts the configuration of a conventional laser apparatus using an external diffraction grating. The laser apparatus 100 includes an FP laser 110, a RF source 120, first and second lenses 130 and 135, an optical coupler 150, and a diffraction grating 160.
The FP laser 110 outputs light modulated based on an electrical signal inputted from the RF source 120. The first lens 130 couples the light outputted from the FP laser 110 into a first optical fiber 141. The first optical fiber 141 is connected to one end of the optical coupler 150, and second and third optical fibers 142, 143 are connected to the other end thereof. The optical coupler 150 transmits the light inputted through the first optical fiber 141 to the second and third optical fibers 142, 143. The light outputted from the second optical fiber 142 is collimated by the second lens 135 to be incident on the diffraction grating 160, and the grating 160 reflects light of a predetermined wavelength. The reflected light is coupled to the second optical fiber 142 through the second lens 135. The optical coupler 150 transmits the reflected light inputted through the second optical fiber 142 to the first optical fiber 141. The reflected light is transmitted through the first optical fiber 141 and is coupled to the laser 110 through the first lens 130. The laser 110 is wavelength-locked by the reflected light, and outputs the wavelength-locked light. By means of the first lens 130 and the optical coupler 150, the wavelength-locked light is transmitted to the third optical fiber 143 for output. The laser device 100 can tune the wavelength of the output light by controlling the reflection wavelength of the diffraction grating 160.
However, the above laser apparatus using the external diffraction grating requires an accurate packaging technology, and the diffraction grating is bulky. Also, applying heat and strain to the fiber Bragg grating for wavelength tuning requires additional devices, and the tunable wavelength range is limited to several nanometers.